The disclosure relates to an electropneumatic field device for controlling a pneumatic actuator that is usually used to adjust a process fluid stream of a technical processing installation like a refinery, a chemical plant, etc. or an installation for manufacturing automation. The disclosure also relates to a method for converting pneumatic energy of an electropneumatic field device into electrical energy.
Today, electropneumatic field devices may have microprocessors and storage elements. Furthermore, additional sensors like a temperature or pressure sensor are often used. In particular, positioners often perform complex calculations: the input and output signals are modified by calculations, complex control algorithms are used, diagnosing parameters are determined and valve movements are captured and stored in order to exactly control a valve position. As opposed to this, IP-converters receive a control current and generate a control pressure signal. Compared to a positioner they lack the feedback of the valve position. A feedback of the pressure signal of IP-converters for the purpose of a more precise control is known but so far is not common in the market.
Electropneumatic field devices are often used in environments exposed to a risk of explosion and therefore mostly use only small amounts of electrical power. Besides, positioners typically need to provide an emergency position to be adopted in case of a power failure. Pneumatic positioners with a spring for storing energy provide a simple, reliable, fast and inexpensive way to realize the emergency position. Field devices with a pneumatic output are therefore common as electrical actuating devices.
In modern applications, a so-called two wire field device is used that receives both electric power and electrical control parameters via two wires. Control information can be transmitted bidirectionally by means of digital protocols, for which field protocols like HART, Profibus or Foundation Fieldbus are used.
Within the domain of electropneumatic field devices, especially of positioners, there is a tendency to increase the functionality of the device with respect to control, monitoring and diagnosis by using further microelectronic components and respective sensors. During the development of such components, it became apparent that the typically available electric power is not or is barely sufficient for devices of higher intelligence and extended functionality, or else a high amount of development effort and investment is required in order to provide the desired sufficient electric power.
Furthermore, in the technical domain of electropneumatic field devices, the tendency can be observed that the communication between control components is realized for example by wireless transmission and a cable connection for power supply is dispensed with. Such a field device without a power supply line requires local energy storage media like batteries, which however have a limited lifetime and thus require safety monitoring.
DE 10 2006 011 503 A1 deals with the above named problem of an insufficient available electric power for the electronic components of electropneumatic field devices. From this publication a valve positioner for pneumatically operated valves of a process automation installation is known. The electropneumatic positioner, which is connected to a pneumatic actuator, comprises an energy converter designed to provide electric power from the compressed air supply stream. The energy converter can be designed with a piezoelement, a plunger coil, or a thermoelement. It became apparent that the known energy conversion systems are only partially applicable in the domain of process technology and/or manufacture automation because the energy provided does not suffice, and the mechanical effort as well as the investment is high for such systems.
An energy conversion unit is generally known from DE 10 2004 04930 A1 in which an oscillating permanent magnet performs a back and forth oscillation by means of a compressed air impulse, wherein the movement of the permanent magnet induces electrical energy in a coil. A compression spring is providing the return force that acts against the pressure impulse. It became apparent that also when using this kind of system the efficiency is insufficiently low. The use of the known energy conversion unit is difficult in process technology and/or manufacture automation, resulting especially to the fact that the mechanical return compression spring as well as the pressure impulse generator must be tuned to each other.